During recovery, transportation and refining of hydrocarbons, the hydrocarbons are associated with other substances that can cause the metallic surfaces of the devices and equipment to undergo corrosion. Generally this is the process by which a corrosive substance reacts with a metal surface, often by oxidation. During oil and gas recovery, this is usually anaerobic corrosion, i.e. only low levels of oxygen are present. A common strategy used to mitigate this problem is to employ corrosion inhibitors.
A critical disadvantage of many commercially available corrosion inhibitors is that they do not meet internationally agreed environmental standards. For example, the Harmonized Mandatory Control System (HMCS) of the Oslo-Paris Convention for the Protection of the North-East Atlantic (OSPAR Convention) requires a standard environmental data-set for each chemical substance, including results for biodegradation, bioaccumulation and aquatic toxicity. The data presented for each substance are compared to a set of criteria that must be met in order for the composition to be permitted for use.
These environmental criteria, and in particular, toxicity to marine algae, are typically not met by many substances used in commercially available corrosion inhibitors. For example, measurement of the effective concentration of the substance that is sufficient to cause a reduction in growth rate for more than 50% of the algae population (termed EC50) is often less than 1 mg/L, and in some cases less than 0.1 mg/L. Ideally, values of EC50 should be >1 mg/L, more preferably >10 mg/L and more preferably higher than this. In other examples, the substances show poor biodegradation, or are considered likely to bioaccumulate.
Consequently a need exists for corrosion inhibitor compositions that are environmentally acceptable but which are also effective corrosion inhibitors. The inventors of the present invention have now found that both of these requirements are met by compositions comprising at least one compound that is a ring-opened derivative of a C5-21 alkylhydroxyethyl imidazoline and a quaternary ammonium compound. Such compositions have been found to have surprisingly low toxicity to marine algae (i.e. surprisingly high EC50 values), acceptable biodegradation and/or acceptable bioaccumulation as well as useful corrosion inhibition properties. One group of compounds that have been found to be particularly useful are commonly called amphoacetates, alkylamidoamineglycinates or amphocarboxyglycinates. Two other related groups of particular interest are known as di-acetates and amphosulfonates. The ring-opened derivatives of C5-21 alkylhydroxyethyl imidazolines have been found to be especially effective when used in combination with alkyl quaternary amines (alkyl quats) and/or alkyl quaternary esteramines (ester quats). Compositions comprising a combination of these compounds exhibit synergy in their corrosion inhibition properties.
The derivatives of C5-21 alkylhydroxyethyl imidazolines described above are known compounds. They are, for example, used in textile wetting agents, hard surface cleaners, shampoos, skin cleansers and personal care products. As far as the applicant is aware, however, there are no commercially available corrosion inhibitors that comprise derivatives of C5-21 alkylhydroxyethylimidazolines as herein described.